The invention relates to a flat lighting device in accordance with the preamble of Claim 1. The invention also relates to a method for operating this lighting device.
More precisely, this is a flat lighting device in the case of which the light of at least one light source is coupled into an optical conductor plate, for example through at least one narrow side (xe2x80x9cedgexe2x80x9d) of the optical conductor plate (so-called xe2x80x9cedge-light techniquexe2x80x9d). By means of reflection, for example at suitable structures on the underside of the optical conductor plate, this light passes through to the outside over the entire front side of the optical conductor plate, and therefore acts as a flat light source which is extended in accordance with the dimensions of the optical conductor plate.
Such lighting devices serve the purpose, for example, of backlighting displays, in particular liquid crystal displays (LCDs), but also large-area advertising panels. Liquid crystal displays have multifarious uses, for example, in control rooms, aircraft cockpits and, increasingly, also motor vehicles, in consumer electronics and as display screens for personal computers (PCs).
Such a lighting device is already known from U.S. Pat. No. 5,408,388. A tubular fluorescent lamp is arrange( parallel to the corresponding closest narrow side on two narrow sides, situated opposite one another, of the optical conductor plate. In order to increase the luminance, and to improve the uniformity, reflecting structures are arranged on the rear side of the optical conductor plate, and diffusely scattering structures are arranged on the front side of the optical conductor plate.
In some cases, for example in darkened rooms and in motor vehicles, aircraft or the like at night, it is necessary for the luminance of the display, and consequently the lighting device, to be lowered, that is to say to be xe2x80x9cdimmedxe2x80x9d.
It is a disadvantage in the prior art that a possibly necessary steep lowering of the luminance cannot be implemented only with the aid of a corresponding lowering of the electric power coupled into the light sources, since in this case either the colour temperature of the light changes too severely, or else the discharge becomes inhomogeneous and the operation of the light source becomes unstable, or the light source even abruptly goes out completely.
It is the object of the present invention to provide a flat lighting device which avoids the disadvantages mentioned and permits the possibility of improved dimming.
According to the invention, a first light source is arranged adjacent to the optical conductor plate of the flat lighting device, and a second light source is arranged downstream of the first light source with reference to the optical conductor plate.
The formulation xe2x80x9c . . . arranged downstream of the first light source with reference to the optical conductor platexe2x80x9d in no way means that the light sources and the optical conductor plate would necessarily have to be arranged in a common plane. Rather, the lamps can also be offset relative to one another and to the optical conductor plate. What is decisive for the advantageous action of the invention is merely that at least a portion of the light emitted by the second light source passes through the first light source into the optical conductor plate. If appropriate, it is also necessary, or at least helpful to have an additional optical system, known per se, which guides the light appropriately through the individual components of the lighting device. The term xe2x80x9cdownstream ofxe2x80x9dxe2x80x94or, from the inverted point of view, the complement xe2x80x9cupstream ofxe2x80x9dxe2x80x94is therefore to be related in this context to the path which the light beams of the second light source cover through the first light source into the optical conductor plate.
For the extreme dimming operation, it is provided to switch off the first light source, and to leave only the second light source switched on or, as the case may be, to switch it on for the first time. For the light of the second light source, which is being coupled into the optical conductor plate, for example through one of its narrow sides, the first light source acts as a diffuser and/or absorber, and thus reduces the fraction of the light flux actually coupled in and, consequently, the luminance of the optical conductor plate.
Suitable in principle as light sources are all elongated electric lamps, in particular elongated fluorescent lamps, for example two tubular fluorescent lamps, which are oriented parallel to one another, or else a single fluorescent lamp having two luminous parts which can be operated independently of one another, for example the two limbs of a U-shaped fluorescent lamp with electrodes which can be switched independently for each limb.
The separate switchability can be implemented with particular ease using dielectrically impeded electrodes, in particular electrodes arranged outside the discharge vessel of the fluorescent lamp.
This type of lamp normally contains in its discharge vessel an inert gas, for example, xenon, as discharge medium. The dielectrically impeded discharge produces excimers, for example, Xe2*, which generate electromagnetic radiation, in particular UV/VUV radiation, which is transformed into visible light by means of fluorescent material(s), if appropriate. Reference is made to the exemplary embodiments for further details on this.
The first light source is preferably provided with a first aperture which, in the simplest case, that is to say without the use of additional optical elements, faces the optical conductor plate. The light is emitted specifically in the direction of the optical conductor plate with the aid of the aperture and, if appropriate, further optical elements. It is possible in this way to achieve an improved efficiency in normal operation.
In the case of the use of a fluorescent lamp, the aperture can be implemented in the simplest case by virtue of the fact that, in part, either the fluorescent material is completely cut out there, or at least thinner layer is applied than in the remaining part of the wall of the lamp.
The measure of the lowering of the luminance by the action of scattering and/or absorption action of the first light source can be influenced by the width of a second aperture, which is arranged along the first light source and faces the second light source. Specifically, the light from the second light source then preferably passes through this second aperture into the first light source and, from there, further on to the optical conductor plate. The second aperture thus acts, as it were, as a slotted stop for the light being coupled into the first light source from the second light source. With the aid of a correspondingly narrow aperture in the first light source, it is thus possible also to use a second light source, which is operated in conjunction with complete electric power injection, to achieve an in principle arbitrarily small luminance of the optical conductor plate.
The second aperture is typically narrower than the first aperture. As a result, a strong scattering, that is to say evident dimming, is achieved on the one hand, and a high efficiency of the first lamp in the undimmed state is achieved, on the other hand.
Moreover, the wall of the lamp can also have a layer for reflecting visible light, the aforementioned apertures being cut out from this reflective layer, or else being at least sufficiently thin in these regions.
The following method steps are proposed according to the invention for the operation, including the dimming operation of the flat lighting device.
For the operation with a high luminance of the optical conductor plate, denoted below as xe2x80x9cnormal operationxe2x80x9d, at least the first light source is switched on, the second light source optionally being switched on in addition. In this operating state, a certain dimming can already be achieved by reducing the power injection into the first light source in a way known per se, this type of dimming, as already mentioned in the beginning, being set certain limits, so that it is not intended for it to be further regarded as a problem here.
For the operation with a luminance of the optical conductor plate which is lower, or even substantially lower, than this, denoted below as xe2x80x9cdimmed operationxe2x80x9d, the first light source is switched off, while the second light source remains switched on or, if appropriate, is switched on for the first time.
Of course, the method according to the invention also provides the possibility of starting directly in dimmed operation, that is to say of immediately switching on the second lamp starting from a state in which both lamps are switched off. In each case, the light flux, being coupled into the optical conductor plate, of the second light source is reduced by the first, switched-off light source, or by scattering and/or absorption. The reduction or scattering action of the first light source, and thus the fraction of light actually coupled in and, finally, the luminance of the optical conductor plate can, as already explained further above, be influenced by the concrete configuration of the first light source, in particular by means of the second aperture.
In a particularly simple design, the second light source is a conventional tubular fluorescent lamp without an aperture. The first light source is preferably an aperture fluorescent lamp.
In addition, it can be advantageous to cover the luminance region between the two states of xe2x80x9cfirst light source onxe2x80x9d and xe2x80x9cfirst light source offxe2x80x9d with the aid of a variable active power injectionxe2x80x94in accordance with the teaching of WO-A-94/23442-, at least into the first lamp. By specific selection of the parameters of xe2x80x9cpulse widthxe2x80x9d and xe2x80x9cpulse durationxe2x80x9d for the pulsed active power injection described in WO-A-94/23442, the light flux both of the first and of the second light source can be influenced.
It is advantageous in this regard that possible inhomogenieties in the luminance distribution of the second light source, such as can occur with the lowering of the electric power injection, play no role, since these are largely homogenized by the scattering action of the first light source.